The present invention relates to an improvement in punch retainers for use in a punch and die assembly.
Punch retainers are used in the prior art to retain a punch in a die shoe when the punch is moved to form an aperture in a sheet of metal. Typically, ball lock punch retainers spring-bias a ball into a notch in the punch. In many prior art retainers a backing plate is attached, suing screws or other non-permanent methods, to a retainer body to dissipate reactive forces from the punch when it is forced into a piece of metal, to form an aperture in the metal. These prior art retainers include a number of passages through which dowels extend to properly align the punch retainer with an upper die shoe of a punch and die assembly. Problems are encountered with this type of prior art retainer since the retainer body is connected to the backing plate after formation of the individual parts. The various passages that extend through the retainer body and the backing plate are often improperly aligned and require close attention by an operator to assemble the punch retainer to a die shoe, which is inefficient. There is often waste since a particular backing plate may not be utilized with a particular punch retainer if the passages in the two can not be properly aligned.
Other problems encountered with the use of backing plates include the fact that the punch is normally slightly smaller in diameter than the punch passage. The passages in the backing plate which are aligned with the punch passage are typically formed in an attempt to center them on the punch passage. As stated above, the prior art cannot always achieve this goal. Even if it did achieve the goal, however, the punch itself is smaller than the punch passage and its center is typically not aligned with the center of the punch passage. Thus, forming the center of the dowel passage in the backing plate such that it is centered with the punch passage does not ensure that the dowel passage is centered on the punch. In fact, it ensures the punch passage is not centered on the punch if the punch retainer is a ball lock type wherein the punch is biased. This creates problems with positioning of the punch, since the dowel passage in the backing plate is used to position the punch retainer on the upper die shoe. In use, a ball biased by a spring typically forces the punch against an outer face of the punch passage such that its center is off-center from the center of the punch passage. Thus, the center line of the dowel passage in the backing plate is typically off-center from the center line of the punch, and the punch is thus not precisely positioned on the upper die shoe.
Further, since the position of the punch on the upper die shoe is determined by the position of the dowel pin extending through the dowel passage, it is important that the dowel passage be accurately positioned relative to the punch. This goal is further made difficult due to the fact that the manufacturing tolerances in forming the punch retainer passage, the dowel passage, the outer diameter of the punch itself, and in securing the backing plate to the retainer body, in combination with the above-discussed problem, all build up to result in a composite inaccuracy that may result in the punch being out of position relative to the upper die shoe by an undesirably large amount. This could result in the parts that are formed by the punch having apertures formed at locations away from a desired location.
Some prior art punch retainers attempt to solve some of these problems by eliminating the backing plate. An example of such a prior art retainer is illustrated in U.S. Pat. No. 3,563,124. In this patent, a plug is utilized in place of a backing plate to dissipate the force received from the punch. The force-dissipating plug must be aligned with the rear of the punch retainer passage so that the reactive force transmitted into the punch will be transmitted into the plug. This patent addressed the alignment problem inherent in the previously discussed prior art by having a dowel aligned with the punch retainer passage extend through the plug and into the die shoe. By eliminating the backing plate, the problem of achieving a number of properly aligned passages through both a retainer body and a backing plate is reduced. A similar device is shown in U.S. Pat. No. 3,589,226.
Problems are still encountered with this type of punch retainer. The forces that must be dissipated from the punch are often of relatively large magnitude, and the plugs disclosed in the above-mentioned patents extend for a relatively small surface area. These plugs sometimes may not adequately dissipate a force, since they do not extend over an adequate surface area.
In addition, prior art punch retainers are also impractical since it is difficult to secure the spring which biases the ball into a spring passage. Special springs are often required which are more expensive than standard springs.
It is an object of the present invention to disclose a punch retainer which utilizes a backing plate such that an adequate surface area is achieved for force-dissipating means; at the same time not requiring alignment of passage within a retainer body and a backing plate when attaching the punch retainer to a die shoe. In addition, the present invention discloses a punch retainer that doe snot require special springs to bias the ball into the punch.